Disqus is a company currently doing hosted commenting systems, primarily for blogs. They should also do these:
* rating blog posts
* polls
* RSVPs
* colored labels (if you see a comment you really like, become a fan of the poster so that their comments are highlighted in the future.
* following (follow that awesome commenter)
* banning (make someone’s comments invisible to you)
that’s it off the top of my head…